This invention relates to the control of emissions resulting from the combustion of sulfur-laden fuels, and, more particularly, to the reduction of sulfuric acid emissions from power plants.
In fossil-fuel power plants, coal, gas, or oil is combusted to heat air, which in turn boils water to form steam. The steam drives a turbine and thence an electric generator, producing electricity. Besides heat, the burning of the fuel produces gaseous pollutants, such as oxides of sulfur and nitrogen. Environmental protection laws mandate that the amounts of gaseous pollutants emitted from the power plant be maintained at acceptably low levels. Additionally, reaction products of the pollutants can damage the power plant apparatus. The present invention deals generally with the technology for controlling and minimizing the emission of sulfuric acid by such power plants.
Fossil fuels, particularly oil and coal, often contain a substantial amount of sulfur. When the fuel is burned in the combustor, the sulfur oxidizes to sulfur dioxide or sulfur trioxide. The sulfur trioxide combines with the moisture of the air to produce sulfuric acid vapor. When the combustion gases cool, either inside or outside the power plant, the sulfuric acid vapor condenses to liquid sulfuric acid. If the condensation is inside the combustion gas ducts of the plant, machinery such as valves, pumps, instrumentation, and pollution control equipment that are contacted by the combustion gas corrode, reducing their operating efficiencies and lives. If the condensation is outside the power plant, there may be environmental damage.
One approach to reducing the undesirable sulfuric acid emission is to inject ammonia into the combustion gas stream. The ammonia reacts with sulfur trioxide and water vapor to produce ammonium sulfate. If there is sufficient water vapor present and the ammonia is provided in an amount of two moles of ammonia for each mole of sulfur trioxide, in theory the sulfur trioxide is completely reacted and removed from the combustion gas stream.
It is important that the proper amount of ammonia be injected. If too much ammonia is injected, an excess of ammonia is present in the power plant emissions, which itself poses an environmental hazard. If too little ammonia is injected, the sulfur trioxide is reacted to ammonium bisulfate rather than ammonium sulfate, which can form a sticky liquid mass inside the ducts or on equipment, or sulfur trioxide is emitted to the atmosphere. Careful control of the ammonia injection mass flow rate is required to avoid any of these undesirable results.
In existing power plants that use this ammonia injection approach, the ammonia content of the gas stream is measured downstream of the ammonia injection location to determine whether excess ammonia is present. If so, the ammonia flow rate is reduced until the excess disappears. This approach has the shortcoming that, even when there is no excess of ammonia, there may still be insufficient ammonia to ensure that the reaction product is ammonium sulfate rather than ammonium bisulfate. Finally, because of system lag times it is often difficult for the instrumentation to follow changes in sulfur trioxide mass flow rates, so that there may be a continuing imbalance that results in either sulfuric acid or ammonia emissions at various times.
There is a need for an improved control approach for use in the emission-control systems of power plants and other facilities that burn fuels having moderate or high sulfur contents. This approach should ensure that neither sulfur trioxide nor ammonia is present in an excess, both during steady state and transient operation, and that sulfur trioxide is reacted to ammonium sulfate rather than ammonium bisulfate. The present invention fulfills this need, and further provides related advantages.